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Old 09-17-2019, 10:21 AM   #1
williebox
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Default n/a k-jet+T not enough fuel under boost

Hey, Im having trouble getting enough fuel under boost, was wondering if there are any more little tricks I could try.

specs: Canadian 76' 242 n/a b21F + 12b turbo, M46, K cam, RSI economy valvetrain kit, 3" downpipe/exhaust, FAST wideband O2, Knocksense knock sensor, G80 LSD, E-fan, amazon BOV. - Used for autox/road course

My K-jet system is pre-lambda, so theres no fuel compensation or boost enrichment available. Ive got the cold start injector wired to a hobbs switch and activating at 0PSI. I have tried shimming the fuel distributor in 0.8mm increments up to 4mm (no more room for more shims so i tried using a solid plug which is ~2mm longer than fully shimmed distributor plug). Each time adjusting the CO screw in the k-jet unit to have an acceptable AFR, but i think i have hit the limits of this very early k-jet system.

The vehicle starts, idles, and mid range RPM is okay, even under full boost (only 5PSI currently), but as soon as im over ~4500rpm it goes lean quick (over 15:1), even when im 10.5:1AFR throughout idle to 3500.

Since there is no boost enrichment from this unit, and no lambda reference, is there anything else I can try to add fuel at higher RPMS?
Some thoughts that come to mind would be replacing the low flowing CSI for a higher flowing unit, or installing a newer style k-jet unit, but I was hoping to race this sunday and I cant think of a quick or easy fix to this issue.
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Old 09-17-2019, 10:27 AM   #2
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The best bet IMO would be turbo injectors and a fuel dist from one. I think the pressures and flow rates are different between NA and turbo.

You may be better off removing a shim or two and lowering your boost so you have “more” fuel at higher RPM (more fuel plate travel at WOT).

Whats your idle afr?
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Old 09-17-2019, 11:05 AM   #3
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Idle AFR on my last test was 10.5:1 so super rich. I tried dialing it back to a reasonable 13.5:1 but it runs out of fuel at a lower RPM when it is set less rich. It almost seems that when the metering plate in the k-jet unit enters the top section of the venturi that's when it starts to lean out quickly. Slow 3rd gear pulls it doesnt go lean as quickly as a full throttle pull in 2nd or 3rd. Theres a turbo k-jet unit a couple hours from where i live for sale that was modded for a VW corrado that i might buy and fiddle with.
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Old 09-17-2019, 11:16 AM   #4
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Quote:
Originally Posted by williebox View Post
Idle AFR on my last test was 10.5:1 so super rich. I tried dialing it back to a reasonable 13.5:1 but it runs out of fuel at a lower RPM when it is set less rich. It almost seems that when the metering plate in the k-jet unit enters the top section of the venturi that's when it starts to lean out quickly. Slow 3rd gear pulls it doesnt go lean as quickly as a full throttle pull in 2nd or 3rd. Theres a turbo k-jet unit a couple hours from where i live for sale that was modded for a VW corrado that i might buy and fiddle with.
I wonder if porting out the fuel dist would help?

I would seriously consider pulling some shims to get off boost AFR good (~13.5), and then use the CSI to compensate under boost. You may be able to swap nozzles on the CSI to improve fueling too.
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Old 09-17-2019, 11:17 AM   #5
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you could also just get a CSI from a v8 Benz or something with bigger displacement.
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Old 09-17-2019, 11:19 AM   #6
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Look up UTCIS, it's a programmable warm up regulator for K-jet, it ain't cheap but it should solve all your problems.
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Old 09-17-2019, 11:40 AM   #7
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Looking into the UTCIS, I had never heard about it previously, seems like a really neat thing. Thanks for the insight, im glad some people stick with the k-jet and are improving tune-ability, Ive got MSII on my 245 and I love the simplicity/finding the balance of k-jet.
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Old 09-17-2019, 12:12 PM   #8
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Quote:
Originally Posted by williebox View Post
Looking into the UTCIS, I had never heard about it previously, seems like a really neat thing. Thanks for the insight, im glad some people stick with the k-jet and are improving tune-ability, Ive got MSII on my 245 and I love the simplicity/finding the balance of k-jet.
Message me to discuss UTCIS. I had it on my Kjet 242 and was not impressed with the support I got from unwired tools.
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Old 09-17-2019, 02:15 PM   #9
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A simple and very effective method would be to do what Volvo did with the B21ET. That is a kjet basic engine and made 155hp. It used a control pressure regulator that has a boost enrichment section in it. So it provides extra fuel under boost.

Along those same lines the Audi 5000 turbo used a boost enrichment control pressure regulator as did the Porsche 930.
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Old 09-18-2019, 03:21 AM   #10
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I know of one person successfully using NA K Jet with a turbo on low boost, though I believe it was the lambda setup. You shouldn't need to change distributors, the stock one flows well enough. As someone else suggested, I would find a turbo CPR for boost enrichment.

Have you done any testing?
Fuel return flow should be 2 L/minute.
With the air flap open completely, injectors should put out 160cc/minute.
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Old 09-18-2019, 08:06 AM   #11
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I havent done any volume tests, I plan on testing everything over the winter while the cars off the road. Im trying to find a local(ish) 240T CPR at the moment, ill try that and go from there.
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Old 09-18-2019, 08:19 AM   #12
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Hi

The metering head of the N/A K-jetronic are pretty much maxed out when reaching 140 crank hp with stock fuel pressure.
A little more can be had by shimming the main fuel pressure regulator.

I would absolutely reccommend you to get hold of a metering unit from a turbo engine.
A shimmed turbo metering unit together with correct warm up regulator should let you run enough boost to get around 200hp safely.

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Old 09-18-2019, 09:29 AM   #13
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Just MS it. Honestly, you'll have more fun that way, and MUCH more control.
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Old 09-18-2019, 10:36 AM   #14
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Quote:
Originally Posted by vwbusman66 View Post
Just MS it. Honestly, you'll have more fun that way, and MUCH more control.
I already have an MS 240, Id like to keep this one factory-ish, im only hoping for around 200hp.

Just wondering has anyone tried an external fuel pressure regulator with boost reference? I know increasing the fuel pressure 1:1 with boost wont add enough fuel at high rpms but it seems like it sure would make adjusting pressure easier and more controllable than adding shims.
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Old 09-18-2019, 11:57 AM   #15
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Quote:
Originally Posted by smoothdurban View Post
Message me to discuss UTCIS. I had it on my Kjet 242 and was not impressed with the support I got from unwired tools.
There support is horrible but even with the base tune it is a huge improvement. warm start is where i have issues now with utcis
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Old 09-18-2019, 12:05 PM   #16
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There support is horrible but even with the base tune it is a huge improvement. warm start is where i have issues now with utcis
I always had problems with na k-jet warm start.
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Old 09-18-2019, 12:21 PM   #17
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Quote:
Originally Posted by mrjaybeeze View Post
Hi

The metering head of the N/A K-jetronic are pretty much maxed out when reaching 140 crank hp with stock fuel pressure.
A little more can be had by shimming the main fuel pressure regulator.

I would absolutely reccommend you to get hold of a metering unit from a turbo engine.
A shimmed turbo metering unit together with correct warm up regulator should let you run enough boost to get around 200hp safely.

YS
Jaybee

This. and measure flow of the injectors...and find the Audi or whatever warm-up regulator with the manifold reference hose connection..

Failing to find one of the rare ancient (and hard to find even 30 years ago) Audi or whatever warm up regulators, you can also gut the electric guts and drill and tap the housing and run a M6 allen that goes in and touches the stainless diaphragm and with that you can vary control fuel pressure...and thereby alter the "fuel slope"....You can make it so it's rich engough at full load under bost but it will be over rich at lower demand..That's just the nature of the beast.
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Old 09-18-2019, 01:43 PM   #18
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These old threads might be helpful:
https://forums.tbforums.com/showthread.php?t=132614
https://forums.tbforums.com/showthread.php?t=168170
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Old 09-18-2019, 07:19 PM   #19
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Quote:
Originally Posted by John V, outside agitator View Post
This. and measure flow of the injectors...and find the Audi or whatever warm-up regulator with the manifold reference hose connection..

Failing to find one of the rare ancient (and hard to find even 30 years ago) Audi or whatever warm up regulators, you can also gut the electric guts and drill and tap the housing and run a M6 allen that goes in and touches the stainless diaphragm and with that you can vary control fuel pressure...and thereby alter the "fuel slope"....You can make it so it's rich engough at full load under bost but it will be over rich at lower demand..That's just the nature of the beast.


Utcis 14.3 in vacuum 10.5 at 15psi. All 84.5 ti parts other than the pressure switch
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Old 09-18-2019, 11:23 PM   #20
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For warm start issues a lot of people overlook the accumulator. It helps a lot with hot starting after the car has been sitting turned off. Anywhere from about 20 minutes to about 40 minutes is the zone where it helps starting.

Another way to add a lot of fuel under boost is to change to a lambda setup with the fuel distributor. That way you have a controller that provides enrichment on two terminals. Terminal 7 is used for boost enrichment on a B12FT. Also on the 82-85 controller is terminal 11 for a very large amount of fuel. It is usually connected to the pressure switch and used for cold accel on the 84-85 B21FT.

One last thing is the early B21F used a 004? injector. You should change to the 015 injector as used on later B21F/B21FT. They probably flow a bit more.
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